The invention relates to gas injection systems, particularly of methane, for internal combustion engines, for the type comprising:
a plurality of electromagnetically controlled injectors associated to the various cylinders of the engine,
a distributing manifold, or rail, communicating with said injectors,
a reservoir feeding the distributing manifold, where the pressurised gas is accumulated,
a pressure regulating valve interposed in the connection between the reservoir and said distributing manifold, and
an electronic control unit set up to control the injectors and to control the opening time to meter the amount of gas injected in each cylinder according to the operating conditions of the engine.
Methane injection systems of the type indicated above have been recently proposed to replace more traditional systems which employ distributing devices to meter the methane to be mixed with the air feeding the engine, according to an arrangement which is similar to that of a normal gasoline carburettor. These more traditional solutions are not acceptable because they are not suitable for working in conjunction with an engine electronic control system which particularly is capable of controlling metering according to the composition of the exhaust gas detected by a lambda sensor. For this reason, the applicant has previously proposed (see European Patent EP 0 801 223) a methane injection system for internal combustion engines comprising a plurality of electromagnetically controlled injectors which are activated by an electronic control unit in order to meter the amount of gas injected in each cylinder according to the operative conditions of the engine.
In the methane electronic injection systems proposed to date, methane metering in the engine cylinders is obtained by varying the opening time of the injectors according to the various operative parameters of the engine, such as the position of the accelerator pedal, the speed of revolution of the engine, the ambient temperature, the altitude (which effects the amount of air taken in by the engine), etc. The opening time of the injectors is the only parameter which can be used to control methane metering, because the pressure of the methane in the distributing manifold is essentially constant.
Furthermore, in said known systems, the pressure regulating valve interposed between the methane reservoir and the distributing manifold essentially consists of a self-standing component, comprising a restricted passage and a membrane mechanism which regulates said passage. However, the systems configured in such a fashion were not satisfying in transient ratios of engine operation. Specifically, the systems are not capable of holding pressure downstream to the valve at a predefined value, which is essentially constant in the case of dynamic variations in the flow of methane, such as, for example, in the case of abrupt engine acceleration. Moreover, the systems are capable of working at a single predefined pressure value to be created in the distributing manifold. In actual fact, the engine operation presents a very high load dynamics which originates methane load variations, also for ratios in the range of 1:30 or 1:40. The pressure regulating valve is usually designed on the basis of a compromise between the existing needs in extreme conditions of load of the engine. This is because a pressure regulating valve with an excessively small passage may be optimal when the engine is idling but will not provide an adequately fast response when the engine load increases rapidly, such as in the case of abrupt acceleration. Conversely, if the passage of the pressure regulating valve is relatively wide, the system will respond well to high engine loads but will not be capable of ensuring accurate metering at low loads.
The object of the invention is to obviate said shortcomings, making a system which is capable of ensuring accurate gas metering in all conditions of operation of the engine and particularly in the case of rapid variations of gas flow determined by abrupt variations of engine load.
In order to achieve this object, the system according to the invention is characterised with respect to the known systems principally in that in comprises means for regulating the pressure of the gas in the distributing manifold.
More precisely, said regulation means comprise:
an electromagnetic actuator controlling said pressure regulating valve,
a sensor of the pressure in the distributing manifold, suitable for sending an electrical signal indicative of said pressure to the electronic control unit,
a sensor of the pressure in the gas feeding line between the reservoir and the pressure regulating valve, suitable for sending a electrical signal indicative of said pressure to the electronic control unit,
memory means associated to said electronic control unit containing maps of the theoretical predetermined pressure values to be created in the distributing manifold according to the variation of the parameters of operation of the engine,
said electronic control unit being programmed to control the electromagnetic actuator of the pressure regulating valve according to the signals output by the sensors of the pressure in the distributing manifold and of the pressure in the line upstream to the pressure regulating valve, in order to obtain a pressure in the distributing manifold which is essentially equal to the theoretical predetermined value that the control unit retrieves in said memory means according to the value of one or more parameters of operation of the engine.
Thanks to the aforesaid characteristics, the system according to this invention is consequently capable of ensuring accurate metering of gas in the cylinders of the engine in any condition of operation of the engine. From a practical point of view, the variation of pressure in the distributing manifold can reach, for example, ratios in the order of 1:3 or higher, for example with a minimum pressure value essentially equal to 3 bars and a maximum value equal to approximately 9 bars. Thanks to the possibility of regulating the pressure value in the distributing manifold, the system is capable of optimally managing any static or dynamic condition of operation of the engine.
Said pressure regulating valve according to the invention has a valve seat and a lock pin moveable between a closed position, in which the lock pin is in contact with the valve seat, and an open condition, in which a restriction is formed between the lock pin and the valve seat putting an inlet opening communicating with the line from the reservoir into communication with an outlet opening communicating with the line leading from the distributing manifold. An important characteristic of the invention is in that said control unit is set up to control a periodical switching of the pressure regulating valve between said closed and open conditions, at a predefined frequency. In other words, alternatively to progressively and continuously varying the passage of the pressure regulating valve according to the electrical control signal sent hereto, the regulation of the pressure in the distributing manifold can also be obtained by opening and closing the valve at a constant frequency. In this case, the regulation of the valve is obtained by varying the duty cycle of the valve, i.e. by varying the ratio between opening time and total period in each opening and closing cycle of the valve according to the pressure upstream and downstream to the valve and according to the conditions of operation of the engine, and particularly to the conditions of static and dynamic load of the engine.
According to an additional characteristic of the invention, said predefined frequency is chosen essentially equal to the frequency at which any injector in the system is activated. The opening of the pressure regulating valve is synchronous and timed in advance with respect to the activation of the injectors, so that said valve opens before each activation of any injection in the system. In this way, the pressure regulating valve also acts as a pre-metering valve, thus performing an important pre-metering function of the fuel immediately before it is injected into the cylinders of the engine via the respective injector.
Thanks to the method described above for regulating the pressure, a valve with a relatively high cross-sectional passage can be used, ensuring in any case precision and promptness of operation in any condition of operation. Furthermore, the choppered control of the pressure regulating valve described above ensures a more robust system for controlling the pressure in the distributing manifold deriving from the capacity of pre-metering the fuel, as mentioned above, before it is injected into the cylinders of the engine.
Additionally, said electronic control unit can be set up to implement said choppered regulation, i.e. by varying the duty cycle of the valve only when the speed of revolution of the engine is under a predetermined value, while for higher engine speeds the control unit is set up to cause a progressive opening of the valve according to the conditions of operation of the engine. An additional advantage of the system according to the invention is that the system is capable of ensuring full operativeness at very low values of the pressure in the methane reservoir 5, i.e. also when the reservoir is nearly entirely empty, this obviously increases the range of the motor vehicle. In this case, the driver may be alerted since when the reservoir is nearly empty the engine performance that can be ensured will obviously be reduced.
Regardless of the injection system described above, the invention also relates to a pressure regulating valve per se, usable in gas injection system of the type comprising a plurality of electromagnetically controlled injectors associated to the various cylinders in the engine, a distributing manifold of the gas to the injectors and a feeding reservoir of the gas to the distributing manifold, where the pressurised gas is accumulated. Said pressure regulating valve comprises, according per se to prior art, a valve body, defining a valve seat, a lock pin moveable between a closed position, in which the lock pin is in contact with the valve seat, and an open condition, in which a restriction is formed between the lock pin and the valve seat putting an inlet opening of the valve seat intended to communicate with a line from the reservoir into communication with an outlet opening communicating with a line leading from the distributing manifold of gas to the injectors, first elastic means for pushing the lock pin towards its closed position and solenoid means for moving the lock pin towards its open position whereby contrasting the action of said first elastic means.
The invention relates to a valve of the type described above, characterised in that said lock pin comprises a stem slidingly mounted in a cylindrical cavity of the valve body and a reduced diameter portion which defines an annular chamber inside said cavity communicating with said inlet opening, so that the pressure in the line communicating with said inlet opening acts on both end surfaces of said annular chamber and consequently does not cause a substantial movement of the lock pin. Always according to the invention, the valve comprises second elastic means tending to move the lock pin towards its open position, said second elastic means having a load lower than that of said first elastic means. The lock pin according to the invention is consequently of the balanced type. An additional characteristic of this invention is in that said stem has a first end facing said solenoid means and a second end defining an auxiliary chamber inside said cylindrical cavity, said auxiliary chamber being in communication with said outlet opening through the passage in the lock pin body. Thanks to this characteristic, the gas which may leak through the clearance between the stem of the lock pin and the aforesaid auxiliary cavity does not exert a force on the lock pin, because discharged in the outlet opening of the valve through said passage in the lock pin body. In a preferred embodiment, the aforesaid lock pin presents an active conical portion co-operating with a valve seat defined by the circular edge of an end opening of said cylindrical cavity. Said solenoid means comprise an electrical winding connected to the valve body and a ferromagnetic material core slidingly mounted in the valve body, with a first end pushed by said first elastic means and a second end which is held in contact with said lock pin in all phases of operation of the valve, by effect of the first and second elastic means.
The pressure regulating valve according to the invention is thus capable of ensuring optimal operation of the system in any condition of operation of the engine. The pressure regulating valve is capable of ensuring gas pressure in the reservoir in the order of 200 bars, gas flows in the order of 40 kg/h with moderate losses of load, not exceeding 10 bars, and is particularly capable of fast response with switching time between the open conditions and the closed condition in the order of hundreds of microseconds (for example, 500 microseconds). This advantage is obtained because, for the aforesaid reasons, the pressure communicated through the inlet opening of the valve does not exert a force on the lock pin, therefore said first elastic means which maintain the lock pin in the normally closed condition can be relatively weak and control the rapid opening and closing of the lock pin with a relatively small, low-power solenoid, which is consequently not very inductive and fast.